An initial step in the fabrication of a printed circuit board or other electronic substrates is the generation of a circuitry pattern from artwork or photographic film onto the substrate. In this method, the substrate containing a metal layer is coated with a protective resist material. The exact pattern desired to be printed in the metal layer is obtained by masking the resist with the desired pattern either mechanically or by use of a phototool. The resist is then exposed to a source of light which polymerizes the uncured resist. The masking or phototool is then removed and the uncured resist is dissolved away, thereby developing the desired image.
The protective resist possesses the ability to give definition and detail required in the circuit. Chemical resistance and dimensional stability are necessary requirements of the resist in order to withstand the subsequent processing steps. Dry film resists are frequently preferred over liquid resists because they offer good chemical resistance, thick plating capability, and cost savings due to less labor and fewer processing steps. Dry film resists find greatest application in the fabrication of printed circuit boards, whereas liquid resists find greatest application in fabrication of integrated circuits and similar applications.
Photosensitive resists are thin coatings produced from organic solutions which, when exposed to light of the proper wavelength, are chemically changed, altering their solubility in certain solvents. These solvents are known as developers. Two types of resists are available: negative acting and positive acting. Negative acting resists are initially soluble in the developer but after exposure to light, become polymerized and insoluble in the developer. Positive acting resists work in the opposite fashion, that is, exposure to light makes the polymer mixture more soluble in the developer. The resist material is frequently dyed to make it visible for inspection and retouching. The pattern that remains after development is insoluble and chemically resistant to cleaning, plating and etching solutions used in the production of the substrate.
The purpose of the developing process is to completely remove unwanted resist in areas to be subsequently etched or plated. Unfortunately, it is impossible in many instances to remove traces of resist during the developing step. This can occur with negative resists which tend to form a thin, cross-linked film at the resist-substrate interface. It is frequently observed that after developing, edges of the resist tend to be somewhat ragged and nonuniform as a result of swelling and deformation during developing. This problem can be overcome to some degree by using a mild plasma treatment after developing and prior to post baking. The purpose of the plasma treatment is to remove in a delicate, almost surgical manner, very tiny quantities of resist in unwanted areas. This procedure, known as plasma descumming, is a commonly practiced step in photolithography operations. The obvious side effects of this step are removal of desired resists and removal of other materials such as exposed metal plating. In thin film applications where the exposed metal plating may only be several hundred .ANG.ngstroms thick, plasma descumming can be detrimental and totally remove the metallization.
The formation of scum or a foot at the base of the resist wall is also thought to be caused by improper curing of the photoresist. After the polymeric resist has been exposed and the latent image has been generated, chemical reactions continue to occur. As a consequence, it is often necessary or advantageous to control the environment of the exposed polymer film in order to allow the desired reactions to continue to completion. Several post-exposure treatments have been reported in the literature, such as thermal treatment, flood exposure with other types of radiation, treatment with reactive gas, and vacuum treatment. Most vendors of photoresist do not encourage the use of any of these types of post-imaging treatments but suggest that the resists simply be held at ambient temperatures for fifteen-to-thirty minutes between the time of imaging and developing. None of the present commercially available photoresist systems utilize post-exposure treatment. One of the primary reasons that resist systems employing complex post-exposure reactions have not gained wide acceptance commercially is that, as a rule, these reactions are difficult to control and reproduce in a production environment. Although the reaction times of the active species are long compared to the exposure times, they are short when compared to the overall process cycle. The reactive sites created in a solid polymer also continue to decay via side reactions. If the exact extent of reaction in the latent image is to be precisely reproduced, it is necessary that the precise degree of chemical reaction be controlled across the substrate and from substrate to substrate.
In order to better illustrate problems obtained with poor definition of the photoresist sidewall, the reader is referred to FIG. 1. A substrate 10 is coated with a photoresist 12 and imaged and developed. Sidewall 14 of the photoresist 12 is not vertical but tapers off at an angle towards the substrate 10. This situation is undesirable. Another type of occurrence found using photoresist is a sidewall 15 that tapers in the opposite direction. This too is clearly undesirable. In FIG. 3, another instance of poorly developed photoresist, the sidewall is relatively vertical but a foot 16 at the base of the sidewall extends out from the photoresist 12 onto the substrate. The foot 16 must be subsequently removed by the plasma descumming operation, however, if the foot is large, descumming is not a viable option.
Clearly, an improved method of processing the photoresist is needed in order to provide the desired vertical sidewalls and to eliminate the foot at the base of the photoresist. At the present time, there are no commercially successful methods for dealing with this problem and secondary operations, such as plasma descumming, are required in order to compensate for this lack of precision. A method that would eliminate undesirable sidewall effects would also eliminate extraneous post-developing steps and be highly sought after by those in the art of photolithography.